A mechanistic investigation of carbon-hydrogen bond stannylation: synthesis and characterization of nickel catalysts

The complex ((i)Pr(3)P)Ni(η(2)-Bu(3)SnCH=CH(2))(2) (1a) was characterized by NMR spectroscopy and was identified as the active species for catalytic C-H bond stannylation of partially fluorinated aromatics, for example in the reaction between pentafluorobenzene and Bu(3)SnCH=CH(2), which generates C(6)F(5)SnBu(3) and ethylene. The crystalline complex ((i)Pr(3)P)Ni(η(2)-Ph(3)SnCH=CH(2))(2) (1b) provides a more easily handled analogue, and is also capable of catalytic stannylation with added Ph(3)SnCH=CH(2) and C(6)F(5)H. Mechanistic studies on 1b show that the catalytically active species remains mononuclear. The rate of catalytic stannylation is proportional to [C(6)F(5)H] and inversely proportional to [Ph(3)SnCH=CH(2)]. This is consistent with a mechanism where reversible Ph(3)SnCH=CH(2) dissociation provides ((i)Pr(3)P)Ni(η(2)-Ph(3)SnCH=CH(2)), followed by a rate-determining reaction with C(6)F(5)H to generate the stannylation products. Kinetic competition reactions between the fluorinated aromatics pentafluorobenzene, 1,2,4,5-tetrafluorobenzene, 1,2,3,5-tetrafluorobenzene, 1,2,4-trifluorobenzene, 1,3,5-trifluorobenzene and 1,3-difluorobenzene all suggest significant Ni-aryl bond formation in the rate-determining step under catalytic conditions. Labelling studies are consistent with an insertion of the hydrogen of the arene into the vinyl group, followed by β-elimination or β-abstraction of the SnPh(3) moiety.     

A divergent strategy for synthesis of the tetrahydroisoquinoline alkaloids renieramycin G and a lemonomycin analog

A divergent synthesis to the tetrahydroisoquinoline alkaloids (±)-renieramycin G and (±)-lemonomycinone amide is reported. A strategy was developed that allowed access to both the diazabicyclo[3.3.1]nonane and diazabicyclo[3.2.1]octane ring systems of the respective targets from a common advanced intermediate. The high diastereoselectivity observed throughout the synthesis is controlled by the first stereocenter formed from alkylation of an unactivated isoquinoline. Key findings include the synthesis of isoquinolines under palladium-free Larock conditions, diastereoselective ionic hydrogenation conditions to set the 1,3-cis-tetrahydroisoquinoline architecture, a highly diastereoselective reprotonation, and a thiophilic Lewis acid-catalyzed 5-endo-trig N-acyl iminium ion silyl enol ether cyclization. This divergent approach to the tetrahydroisoquinoline alkaloids offers an alternative strategy to further structural diversity in this family of natural products.     

Thin-film aerogel thermal conductivity measurements via 3ω

The limiting constraint in a growing number of nano systems is the inability to thermally tune devices. Silica aerogel is widely accepted as the best solid thermal insulator in existence and offers a promising solution for microelectronic systems needing superior thermal isolation. In this study, thin-film silica aerogel films varying in thickness from 250 to 1280 nm were deposited on SiO2 substrates under a variety of deposition conditions. These samples were then thermally characterized using the 3ω technique. Deposition processes for depositing the 3ω testing mask to the sample were optimized and it was demonstrated that thin-film aerogel can maintain its structure in common fabrication processes for microelectromechanical systems. Results indicate that thin-film silica aerogel can maintain the unique, ultra-low thermal conductivity commonly observed in bulk aerogel, with a directly measured thermal conductivity as low as 0.024 W/m-K at temperature of 295 K and pressure between 0.1 and 1 Pa.     

The use of FT-IR as a screening technique for organic residue analysis of archaeological samples

A range of archaeological samples have been examined using FT-IR spectroscopy. These include suspected coprolite samples from the Neolithic site of Catalh?yük in Turkey, pottery samples from the Roman site of Silchester, UK and the Bronze Age site of Gatas, Spain and unidentified black residues on pottery sherds from the Roman sites of Springhead and Cambourne, UK. For coprolite samples the aim of FT-IR analysis is identification. Identification of coprolites in the field is based on their distinct orange colour; however, such visual identifications can often be misleading due to their similarity with deposits such as ochre and clay. For pottery the aim is to screen those samples that might contain high levels of organic residues which would be suitable for GC-MS analysis. The experiments have shown coprolites to have distinctive spectra, containing strong peaks from calcite, phosphate and quartz; the presence of phosphorus may be confirmed by SEM-EDX analysis. Pottery containing organic residues of plant and animal origin has also been shown to generally display strong phosphate peaks. FT-IR has distinguished between organic resin and non-organic compositions for the black residues, with differences also being seen between organic samples that have the same physical appearance. Further analysis by GC-MS has confirmed the identification of the coprolites through the presence of coprostanol and bile acids, and shows that the majority of organic pottery residues are either fatty acids or mono- or di-acylglycerols from foodstuffs, or triterpenoid resin compounds exposed to high temperatures. One suspected resin sample was shown to contain no organic residues, and it is seen that resin samples with similar physical appearances have different chemical compositions. FT-IR is proposed as a quick and cheap method of screening archaeological samples before subjecting them to the more expensive and time-consuming method of GC-MS. This will eliminate inorganic samples such as clays and ochre from GC-MS analysis, and will screen those samples which are most likely to have a high concentration of preserved organic residues.     

LogP prediction performance with the SMD solvation model and the M06 density functional family for SAMPL6 blind prediction challenge molecules

This work presents a quantum mechanical model for predicting octanol-water partition coefficients of small protein-kinase inhibitor fragments as part of the SAMPL6 LogP Prediction Challenge. The model calculates solvation free energy differences using the M06-2X functional with SMD implicit solvation and the def2-SVP basis set. This model was identified as dqxk4 in the SAMPL6 Challenge and was the third highest performing model in the physical methods category with 0.49 log Root Mean Squared Error (RMSE) for predicting the 11 compounds in SAMPL6 blind prediction set. We also collaboratively investigated the use of empirical models to address model deficiencies for halogenated compounds at minimal additional computational cost. A mixed model consisting of the dqxk4 physical and hdpuj empirical models found improved performance at 0.34 log RMSE on the SAMPL6 dataset. This collaborative mixed model approach shows how empirical models can be leveraged to expediently improve performance in chemical spaces that are difficult for ab initio methods to simulate.